This invention relates to the field of tone detectors as for use with stereophonic signals especially and, more particularly, to a detector with improved falsing protection in the presence of noisy signals.
Many tone detectors are known which consist mainly of low-pass or bandpass filters, active or passive, with the output rectified to obtain a DC mode-controlling signal. One such system is shown in U.S. Pat. No. 4,159,398, assigned to the assignee of the present invention. The patent discloses AM stereophonic transmission and reception of a signal including a very low frequency "stereo presence signal" or pilot tone. One problem which can arise with a simple filter/rectifier combination is that false stereo indication signals can be triggered by noise or other signals at or very near the tone frequency. The user of the stereophonic receiver having a falsing problem may see a flickering indicator lamp and hear the audio move back and forth between the "middle" or monophonic position and the separated stereo positions.
A circuit which solved most of this falsing problem has been shown in a co-pending application Ser. No. 218,878, assigned to the assignee of the present invention. That circuit forms the foundation of the present invention as will be apparent in the Detailed Description of the Preferred Embodiment. In general, that circuit must detect a given number of cycles of the desired frequency before signalling a "detect", then must detect a given number of missing cycles before singalling "non-detect". Thus, a missed cycle or two will not affect receiver operation or stereo indication. While this prior circuit is completely workable, it is possible to receive a number of false detects from the signals being received if the desired signal is very weak and there is co-channel interference.